A widely used hormone replacement therapy (HRT) formulation (~ 57 million prescriptions in 2003 alone) contains the conjugated equine estrogens equilenin (EN), equilin, and 8,9-dehydro-estrone (total ~ 54% composition), and the endogenous estrone and 1713-estradiol. Although the long term risks of cancers of the breast and other hormone-sensitive tissues have been well publicized, women continue to use this formulation because of the significant health benefits associated with the relief of post-menopausal systems. The metabolic activation of equine and endogenous estrogens to genotoxic metabolites has been implicated in the etiology of cancers associated with HRT. It is known that the catechol 4-hydroxyequilenin (4-OHEN) is the most significant metabolite among all three equine estrogens, and that it forms unusual cyclic, stable DNA adducts in vitro, in rat mammary tissue model systems, and in human breast tissue. Adducts of 4- OHEN with cytosine, adenine, and guanine in DNA have been recognized, and each type of adduct is characterized by four stereoisomedc forms. If not removed by cellular DNA repair mechanisms, such adducts may be incorrectly replicated by DNA polymerases, thus causing mutations that may ultimately lead to the development of tumors. Nucleotide excision repair (NER) is the major repair mechanisms that removes bulky adducts from DNA in human cells with efficiencies that depend on the structural properties of the adducts and the distortions they cause in the DNA structure. However, there is no information on how the 12 different 4-OHEN-DNA adducts are processed by NER enzymes. The goal of this project is to determine how the conformations of the different stereoisomeric forms of these adducts influence the efficiencies of their removal by human NER enzymes. In aim 1, site-specific oligonucleotides with single 4-OHEN-DNA lesions will be constructed. In aim 2, their structural features will be analyzed by high resolution NMR and computational techniques, while in aim 3, the resistance to DNA repair will be evaluated using cell free extracts in cells, and in several different types of cells in culture treated with 4-OHEN.The identification of the 4-OHEN-DNA adducts that are resistant to DNA repair in human cells is significant because this information could help to (1) develop biomarkers for identifying women at risk to the adverse effects of HR7, and (2) stimulate the design of new, modified estrogen replacement drugs that are less resistant to nucleotide _,xcision repair enzymes at the DNA adduct level, and thus less active as potential cancer initiating agents.